Bio-based flame retardant prepared from chitosan and sodium citrate: Enhancing fire safety, transparency, and mechanical properties of thermoplastic polyurethane elastomer

Thermoplastic polyurethane elastomer (TPU) is a common and widely used material with flammable characteristics, which poses a great safety hazard. Chitosan (CS) is a renewable bio-based flame retardant. Sodium citrate (CNa) is an organometallic salt that can alter the thermal degradation behavior of TPU. Based on existing research, by integrating the two substances, the newly developed bio-based, eco-friendly flame retardant (CS@CNa) is capable of substantially boosting the flame retardant and smoke suppressant characteristics of TPU composite materials. Relative to pure TPU, the peak heat release rate (pHRR) and peak smoke release rate (pSPR) of the TPU/CS@CNa6 % composite dropped by 71.92 % and 60.94 %, respectively. Additionally, the limiting oxygen index (LOI) of TPU/CS@CNa6 % reached 27 %, and it secured a V-0 rating in UL-94 testing. These results indicate that CS@CNa can significantly improve the flame retardancy of TPU. Moreover, CS@CNa demonstrates a beneficial effect on both the transparency and mechanical performance of TPU composites. With increase in amount of CS@CNa added gradually increases, TPU/CS@CNa composite samples still maintain clear visual effects. TPU/CS@CNa6 % exhibits a tensile strength of 22.61 MPa and an elongation at break of 977.69 %, representing increases of 139.3 % and 157.2 %, respectively, compared to pure TPU. This work exhibits great significance for manufacturing of multifunctional flame retardant TPU composites and expansion of TPU application scenarios.